I'm also wondering if a re-clustering of the table would work based on
the index that's used.
such that:
CLUSTER core_object USING plugins_plugin_addr_oid_id;
and see if that makes any change in the differences that your seeing.
On 04/13/2010 02:24 PM, Kevin Grittner wrote:
> norn <andrey.perliev@gmail.com> wrote:
>
>
>> I am wondering why there are so big gap between two limits and how
>> to avoid this...
>>
>
> I think we've already established that it is because of the
> percentage of the table which must be scanned to get to the desired
> number of rows. The problem is exacerbated by the fact that it's a
> "backward" scan on the index, which is slower than a forward scan --
> mainly because disks spin in one direction, and the spacing of the
> sectors is optimized for forward scans.
>
> There are a couple things to try which will give a more complete
> picture of what might work to make the run time more predictable.
> Please try these, and run EXPLAIN ANALYZE of your problem query each
> way.
>
> (1) Try it without the ORDER BY clause and the LIMIT.
>
> (2) Temporarily take that top index out of consideration. (Don't
> worry, it'll come back when you issue the ROLLBACK -- just don't
> forget the BEGIN statement.)
>
> BEGIN;
> DROP INDEX plugins_plugin_addr_oid_id;
> explain analyze <your query>
> ROLLBACK;
>
> (3) Try it like this (untested, so you may need to fix it up):
>
> explain analyze
> SELECT core_object.id
> from (SELECT id, city_id FROM "plugins_guide_address")
> "plugins_guide_address"
> JOIN "plugins_plugin_addr"
> ON ("plugins_plugin_addr"."address_id"
> = "plugins_guide_address"."id")
> JOIN "core_object"
> ON ("core_object"."id" = "plugins_plugin_addr"."oid_id")
> WHERE "plugins_guide_address"."city_id" = 4535
> ORDER BY "core_object"."id" DESC
> LIMIT 4 -- or whatever it normally takes to cause the problem
> ;
>
> -Kevin
>
>